[Spread of Optical Access Networks]
Along with the spread of the Internet, recently there has been an increasing need for high-speed network operation and usage of ADSL (Asymmetric Digital Subscriber Line), and B-PON (Broadband PON), E-PON (Ethernet PON), G-PON (Gigabit Capable PON) is becoming more widespread. The PON or passive optical network system in particular when connected between a network unit (ONU: optical network unit) installed in each user's home and the relay station (OLT: Optical line terminal) installed in each station connects each user via an optical splitter coupled to a single optical fiber from the OLT. This arrangement allows a low fiber installation cost as well as high-speed communication utilizing optical transmission so that usage is currently spreading widely throughout the world.
[TDM-PON System]
Among the PON systems, the TDM-PON system (Time Division Multiplexing) is utilized at different light wavelengths for downstream transmissions from the OLT to the ONU, and upstream transmissions from the ONU to OLT for time-divided signal sharing among the ONU (user units). The TDM-PON system is utilized in the B-PON, E-PON, G-PON, 10G-EPON, and XG-PON systems.
[DBA Control of TDM-PON]
In TDM-PON operation, the OLT manages the timing that optical signals are sent from the ONU in order to prevent conflicts between optical signals from the ONU in upstream transmissions. More specifically, the OLT sends a control frame to each ONU instructing at what periods transmit is allowed and ONU sends upstream control frame and upstream data during the period where transmission is allowed. Based on the volume of frame data received from the coupled terminals, the ONU sends a control frame to the OLT requesting a bandwidth space. Generally, DBA (Dynamic Bandwidth Allocation) control is utilized to dynamically manage the periods where transmit by the ONU is allowed based on the bandwidth space requested by the ONU.
Different types of control methods relating to this DBA are known in the art. The method in Japanese Unexamined Patent Application Publication No. 2009-152706 for example discloses a method that sets the DBA sequence so that the difference in reverse bias voltage becomes smaller, and regulates the reverse bias voltage in synchronization with the timing of transmissions from the ONU.
[WDM/TDM-PON System]
Rather than 10G-EPON or XG-PON, the WDM/TDM-PON systems that group TDM-PON of the related art into a plurality of wavelengths are considered as a more likely potential candidate for next-generation passive optical networking (PON). Technology relating to WDM/TDM-PON is disclosed for example in Japanese Unexamined Patent Application Publication No. 2011-55407 and S. Kimura, “10-Gbit/s TDM-PON and over-40-Gbit/s WDM/TDM-PON systems with OPEX-effective burst-mode technologies, “OFC2009, OWH-6, March 2009.
[Need for Multi-Branching and Line Extension in TDM-PON]
Techniques for multi-branching and line extension in TDM-PON are known to include utilizing optical amplifiers at the OLT for high output at the optical transmitter section and for high-sensitivity at the optical receiver section. Signals transmitted upstream in TDM-PON are burst signals whose amplitude widely fluctuates from strong to weak. The cause of the widely fluctuating strong and weak signals is the different distances between the OLT and each ONU, which consequently cause different optical loss in the optical fiber between allowable transmission loss in the PON zone. A method is disclosed in Z. BelFqih et al, “Enhanced Optical Budget System Performance of a Burst Extended PON at 10.7 Gbps over 60 km of Fibre”, ECOC2008, Th2. F. 4. (2008), for example for expanding the allowable transmission loss by utilizing optical amplifiers in the G-PON.